In a transmission process, signals are reflected, refracted, and diffracted due to an influence of terrain and features, causing the signals being propagated along different paths. This is called multipath propagation. The multipath propagation makes signals received at the receiving end with different amplitudes, phases, frequencies, and arrival times. That is, multipath signals obtained through the multipath propagation have different fading characteristics. Multiple-input multiple-output (MIMO) technologies in traditional terrestrial wireless communication take advantage of the characteristics that signals will have different fading characteristics due to the influence of terrain and features in a transmission process, and reduce a correlation between receiving antennas at a terminal side by increasing antenna spacing, ensuring that the fading characteristics of the multipath signals received by different antennas are different. Thereby, spatial division multiplexing is realized to improve a downlink throughput. In an ideal case, the distance between the receiving antennas may be any distance which is greater than half the wavelength.
However, the above-mentioned space division multiplexing is no longer applicable in some specific communication environments, such as a light-of-sight communication environment. Between the transmitting end and the receiving end of the signal in the light-of-sight communication environment, there is no obstacle causing a propagation path being changed, and signals arrive at the receiving end in a single path without differences in amplitude, phase, frequency, and arrival time. Therefore, in such an environment, using the method of reducing the correlation between receiving antennas to ensure that the signals are received with different fading characteristics is no longer applicable. In such an environment, of course, conventional methods for the spatial division multiplexing maybe useless.
Typical light-of-sight communication environment is common, and the most typical one may be air-to-ground (ATG) communication. In an ATG system, a flying height for an aircrafts is about 10,000 m, which allows a light-of-sighting communication with a radius about 400 km. Thus, the light-of-sighting communication may be the main form of communication between a base station and an airborne station. In addition to water surfaces, the surfaces are mainly diffuse reflection surfaces in this scenario, thus few reflected, refracted, and diffracted multipath signals may arrive at the airborne station. In other words, the correlation between receiving antennas is strong, and even if the spacing between two antennas of the airborne station is increased to dozens of meters, which is greater than 10 times the wavelength and far exceeds the general requirement of macrocells for terrestrial mobile communication systems (i.e., the distance between receiving antennas should be more than half the wavelength), it is still not possible to ensure the different fading characteristics of signals. Therefore, the conventional method of reducing the correlation between MIMO antennas is useless in the light-of-sight environment, and the method for spatial multiplexing is not suitable for light-of-sight environments.